1. Field of the Invention
A present invention relates to an electrostatic micro switch which performs switching by drive of electrostatic attraction, an electrostatic micro switch production method, and an apparatus provided with the electrostatic micro switch.
2. Description of the Related Art
An RF-MEMS (Radio Frequency Micro Electro Mechanical Systems) element which is of a conventional electrostatic micro switch will be described below with reference to FIG. 20 to FIG. 26.
FIGS. 20A and 20B show an outline of the RF-MEMS element. A RF-MEMS element 81 of FIG. 20 functions as a switching element of a coplanar line while incorporated into a high-frequency circuit. The RF-MEMS element 81 has a substrate 82. A coplanar line (CPW line) 83 which is of a line for transmitting a high-frequency signal is formed on the substrate 82. In the coplanar line 83, a signal line 83s is located between two ground lines 83g1 and 83g2 at certain intervals.
A movable body 84 is provided in the substrate 82. The movable body 84 is arranged above the coplanar line 83 at certain intervals while commonly facing the signal line 83s and parts of the ground lines 83g1 and 83g2 of the coplanar line 83. The movable body 84 is supported by the substrate 82 through beams 85 and support portions 89 such that displacement is vertically allowed with respect to the substrate 82. A movable electrode 86 is formed on a surface on the side of the substrate 82 in the movable body 84.
FIG. 21A simplistically shows an example of an arrangement relationship between the movable electrode 86 and the coplanar line 83 when viewed from above the RF-MEMS element 81, and FIG. 21B shows an example of the arrangement relationship between the movable electrode 86 and the coplanar line 83 when laterally viewed. As shown in FIG. 21, the movable electrode 86 is formed so as to stride across the ground line 83g1, the signal line 83s, and the ground line 83g2 of the coplanar line 83, and the movable electrode 86 faces the lines 83s, 83g1, and 83g2 while separated from the lines 83s, 83g1, and 83g2 at certain intervals.
Returning to FIGS. 20A and 20B, a protection insulating film 87 is formed on a surface of the movable electrode 86. In the substrate 82, a fixed electrode for moving 88 (88a and 88b) is formed in a region which faces the movable body 84.
In the MEMS element 81 having the above configuration, movable body displacing means for displacing the movable body 84 is formed by the movable body 84 which is of the electrode and the fixed electrodes for moving 88a and 88b. When a direct-current voltage is applied between the movable body 84 and the fixed electrode for moving 88 from the outside, electrostatic attraction is generated between the movable body 84 and the fixed electrode for moving 88. As shown in FIG. 20B, the movable body 84 is attracted toward the side of the fixed electrodes for moving 88 by the electrostatic attraction. Thus, the movable body 84 can be displaced by utilizing the electrostatic attraction with the movable body 84 and the fixed electrode for moving 88. The displacement changes an electrostatic capacitance between the movable electrode 86 and the coplanar line 83, which allows to signal conduction to be turned on and off in the coplanar line 83.
Because the MEMS element 81 having the above configuration is formed by a MEMS technology, the small, low-loss electrostatic micro switch having good high-frequency (transmission) characteristics can be realized.
The movable body 84 is made of a high-resistance semiconductor whose resistivity ranges from 1 kΩcm to 10 kΩcm. The high-resistance semiconductor shall mean a semiconductor which behaves as an insulating material for the high-frequency signal (for example, signals having frequencies not lower than about 5 GHz) while behaving as the electrode for a low-frequency signal (for example, signals having frequencies not more than about 100 kHz) and a direct-current signal. That is, the movable body 84 made of the high-resistance semiconductor has good dielectric-loss characteristics for the high-frequency signal, whereas the movable body 84 functions as the electrode for the direct-current signal (direct-current voltage).
There are the following problems in the conventional electrostatic micro switch. When the direct-current voltage is applied between the movable body 84 and the fixed electrode for moving 88 to displace the movable body 84, a depletion layer 90 (90a and 90b) is formed in a region of the movable body 84, where the movable body 84 faces the fixed electrode for moving 88.
The above phenomenon will be described in detail with reference to models shown in FIGS. 22 and 23. FIGS. 22A and 23A show models in which counterparts of the movable body 84 and the fixed electrode for moving 88 are modeled as a capacitor, and FIGS. 22B and 23B show equivalent circuits of the models respectively. In the models, a gap 91 located between the movable body 84 and the fixed electrode for moving 88 is an insulator and the movable body 84 is the semiconductor. Therefore, the models have a MIS structure (Metal Insulator Semiconductor) structure which is one of modes of the transistor.
FIGS. 22A and 22B show the state in which the direct-current voltage is not applied between the movable body 84 and the fixed electrode for moving 88. In this case, as shown in FIG. 22B, a total capacitance C of the capacitor is equal to a capacitance Co of a capacitor which is formed through the gap 91 by the movable body 84 and the fixed electrode for moving 88.
On the other hand, FIGS. 23A and 23B show the state in which the direct-current voltage is applied between the movable body 84 and the fixed electrode for moving 88. In this case, as shown in FIG. 23A, the depletion layer 90 is formed in the region of the movable body 84, where the fixed electrode for moving 88 faces the movable body 84 made of the semiconductor. This leads to the state in which the new capacitor is formed in the movable body 84, and the new capacitor and the capacitor formed through the gap 91 are connected in series as shown in FIG. 23B. Accordingly, the total capacitance of the capacitor becomes 1/C=(1/Co)+(1/Cs) and the total capacitance is decreased, so that the voltage at the gap 91 is decreased.
An expression in which the capacitance C of the MIS structure shown in FIGS. 22 and 23 is normalized by the capacitance Co is obtained as follows:
                    [                  Expression          ⁢                                          ⁢          1                ]                                                                      1          C                =                              1                          C              o                                ⁢                      {                          1              +                                                                                          2                      ⁢                                                                                          ⁢                                              ɛ                        0                                            ⁢                                              ɛ                        o                        2                                                                                                            qN                        a                                            ⁢                                              X                        o                        2                                            ⁢                                              ɛ                        Si                                                                              ⁢                  V                                                      }                                              (        1        )            
Where ∈0 is a dielectric constant of vacuum, ∈o is a dielectric constant of an insulator, q is a charge amount of electron, Na is a carrier concentration, Xo is a thickness of an insulator, ∈Si is a dielectric constant of a semiconductor, and V is an applied voltage.
FIG. 24 shows a relationship between the ratio of C/Co and the applied voltage when the resistivity of a silicon semiconductor is variously changed based on the above expression (1). Referring to FIG. 24, it is found that the ratio of C/Co is decreased as the semiconductor resistivity is increased. That is, when the resistivity is high, the depletion layer is increased and the capacitance Cs is also increased. Therefore, the voltage drop at the gap 91 by the capacitance Cs is increased as the resistivity is increased. Accordingly, in order to perform the desired operation of the movable body 84 which is of the high-resistance semiconductor, it is necessary that the high direct-current voltage be applied between the movable body 84 and the fixed electrode for moving 88 when compared with the case where the movable body 84 is made of the low-resistance semiconductor.
FIG. 25 shows the equivalent circuit of the state in which a direct-current power supply 92 applies the voltage between the movable body 84 and the fixed electrode for moving 88. In FIG. 25, R is a resistance of the movable body 84, vc is a terminal voltage of the capacitor, vR is a terminal voltage of the resistance, and ic is a current passed through the movable body 84.
Because the circuit shown in FIG. 25 becomes an RC circuit, the following expression holds.
                    [                  Expression          ⁢                                          ⁢          2                ]                                                                      v          C                =                  V          ⁡                      (                          1              -                              ɛ                                  -                                      t                    CR                                                                        )                                              (        2        )            
Where ∈ is a base of a natural logarithm and t is time. As can be seen from the expression (2), the time t during which the voltage vc is brought close to the applied voltage V is lengthened, when a product of the resistance R and the capacitance C is increased.
FIG. 26 is a graph showing the relationship between resistance R and time t, in which a terminal voltage vc of the capacitor becomes V, when the capacitance C of the capacitor is set at 1 μF in the equivalent circuit shown in FIG. 25. As can be seen from FIG. 26, a charging time to the capacitance is lengthened as the resistance R is increased. That is, the charging time to the capacitor is lengthened, when the resistivity of the semiconductor which is of the movable body 84 is increased.
When the direct-current voltage is applied between the movable body 84 and the fixed electrode for moving 88, the movable body 84 is brought close to the fixed electrode for moving 88, which increases the capacitance C of the capacitor. Therefore, the charging time to the capacitor is further lengthened, which decreases an operation speed of the electrostatic micro switch.
In order to avoid the above problems, it is thought that the resistivity of the movable body 84 is decreased. However, in this case, transmission characteristics of the high-frequency signal are lowered.